Devices for the movement of gases are widely utilized. The very first aircraft engines were piston driven propellers. They worked by coupling a piston engine to a propeller. This simplicity led to widespread adoption until jet engines were invented. Turbojet engines work by the principle of coupling a turbine to a fuel combination system. Spinning of the turbine compresses a fuel-air mixture which, when burned, provides thrust and torque to rotate the turbine. The first turbojet engines derived their thrust from exhaust leaving the engines. Modern variants of the turbojet engines include turbo prop and turbofan engines, which use torque generated by the exhaust to drive a propeller or fan in addition to compressing the fuel-air mixture. Rocket engines are possibly one of the oldest mechanical propulsion systems, and have not changed much since their inception. A rocket comprises a tube or cone in which sits (or into which is fed) a fuel oxidizer mixture. Expanding gas from combustion of this mixture creates thrust. Rockets, while offering the highest fuel-thrust ratio of any existing propulsion systems, cannot easily vary the amount of thrust they generate. Even adding an ability to turn a rocket on or off significantly complicates its design.
The ability of a temperature differential to drive gas flow at a surface has long been known. In 1873, Sir William Crookes developed a radiometer for measuring radiant energy of heat and light. Today, Crookes's radiometer is often sold as a novelty in museum stores. It consists of four vanes, each of which is blackened on one side and light on the other. These are attached to a rotor that can turn with very little friction. The mechanism is encased inside a clear glass bulb with most, but not all, of the air removed. When light falls on the vanes, the vanes turn with the black surfaces apparently being pushed by the light.
Crookes initially explained that light radiation caused a pressure on the black sides to turn the vanes. His paper was originally supported by James Clerk Maxwell, who accepted the explanation as it seemed to agree with his theories of electromagnetism. However, light falling on the black side of the vanes is absorbed, while light falling on the silver side is reflected. This would put twice as much radiation pressure on the light side as on the black, meaning that the mill is turning the wrong way for Crooke's initial explanation to be correct. Other incorrect explanations were subsequently proposed, some of which persist today. One suggestion was that the gas in the bulb would be heated more by radiation absorbed on the black side than the light side. The pressure of the warmer gas was proposed to push the dark side of the vanes. However, after a more thorough analysis Maxwell showed that there could be no net force from this effect, just a steady flow of heat across the vanes.
The correct explanation for the action of Crookes radiometer derives from work that Osborne Reynolds submitted to the Royal Society in early 1879. He described the flow of gas through porous plates caused by a temperature difference on opposing sides of the plates which he called “thermal transpiration.” Gas at uniform pressure flows through a porous plate from cold to hot. If the plates cannot move, equilibrium is reached when the ratio of pressures on either side is the square root of the ratio of absolute temperatures.
Reynolds' paper also discussed Crookes radiometer. Consider the edges of the radiometer vanes. The edge of the warmer side imparts a higher force to obliquely striking gas molecules than the cold edge. This effect causes gas to move across the temperature gradient at the edge surface. The vane moves away from the heated gas and towards the cooler gas, with the gas passing around the edge of the vanes in the opposite direction. Maxwell also referred to Reynolds' paper, which prompted him to write his own paper, “On stresses in rarefied gases arising from inequalities of temperature.” Maxwell's paper, which both credited and criticized Reynolds, was published in the Philosophical Transactions of the Royal Society in late 1879, appearing prior to the publication of Reynolds' paper. See, Philip Gibbs in “The Physics and Relativity FAQ,” 2006, at math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html.
Despite the descriptions by Reynolds and Maxwell of thermally driven gas flow on a surface dating from the late 19th century, the potential for movement of gases by interaction with hot and cold surfaces has not been fully realized. Operation of a Crookes radiometer requires rarefied gas (i.e., a gas whose pressure is much less than atmospheric pressure), and the flow of gas through porous plates does not yield usable thrust, partially due to the thickness and due to the random arrangement of pores in the porous plates.
Thermal transpiration refers generally to the formation of a pressure gradient in gas inside a tube, the pressure gradient formed when there is a temperature gradient in the gas inside the tube, and when the mean free path of the molecules in the gas is a significant fraction of the tube diameter.
Construction of a thermal transpiration device to operate at 1 ATM (standard atmosphere pressure) is difficult as, optimally, the hot and cold sides must be within 100 nm or less of each other. A 100 nm thick film exposed to an unfiltered, uncontrolled environment tends to be too fragile to withstand typical environmental stresses, such as, for example, impact from debris and/or handle the sheer forces produced by changes in air current.
Furthermore, the only insulation that is generally efficient at that scale is a vacuum. This means that that if the Bernoulli effect is used to draw a vacuum between the two membranes, at least one of the membranes used to form the thermal transpiration device must be thinner than 50 nm. Such a thin membrane would not last long due to the typical environmental stresses placed on the device when in use.
Thus there is a need for a way to optimize the thermal transpiration/radiometric effect described above for practical uses.